The cephalosporin antibiotics cephalothin and cephaloridine can produce nephrotoxicity in man. Ongoing studies in this laboratory concerned with the molecular pharmacology of this nephrotoxicity have required the use of selectively labeled cephalothin and cephaloridine. We have successfully prepared cephalothin-H3 and cephaloridine-H3 with randomly labeled gamma-thienylacetic acid. However, because of the facile exchange of tritium with these compounds, the corresponding cephalothin-C14 and cephaloridine-C14 are currently being synthesized from gamma-thienyl acid-1-C14. A small quantity of cephaloridine-(2,6-pyridine-C14), has also been prepared.